Herpes simplex virus type 1 (HSV-1) infected cell polypeptide 4 (ICP4) is a critical regulator of viral gene expression that is required for productive infection. ICP4 has been shown to act, depending on the promoter structure, as both an activator or repressor of viral genes. ICP4 has two broad transcriptional regulatory domains, the N-terminal domain, which exhibits both repression and transactivation, and the C-terminal domain, which is involved exclusively in transactivation. ICP4 regulates transcription through interactions between ICP4's transcriptional regulatory domains and cellular general transcription factors including components of TFIID. Although it has been shown that a region in the amino-terminus of ICP4 corresponding to aa 30-210 is necessary for appropriate transactivation and repression of viral genes, specific domains responsible for these activities remain uncharacterized. Using deletion mutants spanning this region, we show that the entire region is necessary for ICP4 function, but that loss of aa 30-142 has a greater detrimental impact on the ability of ICP4 to transactivate E and L genes. Neither deletion had a significant impact on ICP4 repression, however. The protein interactions made by ICP4 in vivo during infection were also studied. We show ICP4 forms complexes with TFIID, thus verifying previous in vitro data. Novel interactions between TFII-I and components of the Mediator complex were also identified. In addition, Mediator was found to colocalize with ICP4 starting at early and continuing into late times of infection. Mediator was also recruited to viral promoters in an ICP4-dependent manner, showing a direct role for Mediator in ICP4-mediated transcription. Together, the data show that ICP4 regulates transcription through interactions between its complex regulatory domains and a diverse set of cellular protein complexes, including components of the basal transcription machinery and coactivator complexes
